The present invention pertains to the manufacture of drill bits, and especially to drag-type bits of the type wherein a number of relatively small cutter devices or cutters are brazed into a bit body. Such drag-type bits, which may be either full bore or core type, typically have bit bodies comprised of one or more body members either of steel or of a tungsten carbide matrix material. Brazing may be used as a means of mounting cutter devices in either type of bit body. However, since it is relatively easy to interference fit the cutter devices into a steel bit body using more or less conventional techniques, but is difficult to achieve such interference fits with matrix bit bodies, the present invention may, in that sense, be considered especially suitable for the manufacture of matrix body bits.
More specifically, the cutter devices to be mounted on the bit body each typically comprise a mounting body or "post" including a stud portion to be mounted in a respective pocket in a bit body member and defining one end of the cutter device. The cutter device has a cutting formation generally adjacent the other end of the mounting body or post. Even more specifically, the post may be comprised of sintered tungsten carbide, and the cutting formation may be defined by a layer of polycrystalline diamond material carried on the post, either directly or via an intermediate carrier member, e.g. a disc of sintered tungsten carbide. However, the present invention could be applied to other forms of cutters, e.g. monolithic bodies.
A number of problems have been associated with past techniques for brazing such a stud portion into its respective pocket in the bit body member. For various reasons, there have been instances in which the braze material has not completely filled the void areas or space between the stud portion and the pocket. This may render the resulting product more susceptible to breakage or failure in use.
In most bit designs, each cutter device and its respective pocket has a forward or leading side, which can be determined from the direction in which the bit will rotate in use, and on which leading side the aforementioned cutting formation is generally located. The opposite side of the cutter device and pocket may be considered the rear or trailing side. In use, high forces are exerted on the cutter in a forward to rear direction. Thus, the aforementioned failures and breakages are a particular problem if there is a gap, unfilled by braze material, between the pocket and the stud portion of the cutter device at the trailing side and near the mouth of the pocket.
These and other problems are at least partially due to the fact that, as a bit body member, with cutter studs emplaced at various pockets, is heated during the brazing process, the exterior heats faster than the interior. Thus, a temperature gradient is established with temperature decreasing inwardly from the exterior of the bit body.
It is not practicable to completely fill the void areas between each cutter stud portion and its respective pocket with braze material prior to heating. Because the heating is preferably done in a controlled, i.e. reducing, atmosphere in a closed chamber, a human operator cannot add to or manipulate the brazing material as it is heated. Thus, a body of brazing material, e.g. in the form of a ring surrounding the cutter, has been placed adjacent the mouth of the pocket prior to heating. In some cases, a thin strip of brazing material was also placed in the pocket in an effort to define a flow path tending to draw brazing material from the main body down into the pocket.
With such a technique, when the main body of brazing material at the mouth of the pocket reaches its melting point, it may begin to flow downwardly into the pocket, but due to the aforementioned temperature gradient, may refreeze before it reaches the bottom of the pocket and completely fills the void areas. Even if the material is able to flow all the way to the bottom of the pocket, it is still difficult to ensure that it will completely fill all the void areas across the bottom of the pocket and about the sides of the stud portion of the cutter. These problems may be aggravated by the fact that, if the body of brazing material adjacent the mouth of the pocket reaches its melting point but does not flow into the pocket, the more volatile components of the brazing material may evaporate, thus raising the melting point of the remainder, and perpetuating the aforementioned problem caused by the temperature gradient.
Still another problem is that the brazing material which is placed near the mouth of the pocket may run off across the face of the bit body member, and be wasted, rather than flow into the pocket.
Yet another problem is that, even if the void areas between the stud and pocket are completely filled by braze material initially, this material may be eroded away by the drilling fluid in use. Once again, creation of a gap near the mouth of the pocket on its trailing side is particularly disadvantageous.
Another somewhat different problem is that of providing a simple, yet effective means of temporarily retaining the stud portion of the cutter assembly in its pocket while the brazing is performed. In some cases, where it has been possible to get a substantial amount of brazing material to enter the pocket, the molten brazing material may tend to push the stud portion outwardly from the pocket with a sort of "percolating" effect.